cauda equina

**Anatomy** - The adult spinal cord ends at ~L1–L2 (the conus medullaris). In newborns it ends at ~L3. Below this, the lumbar, sacral, and coccygeal nerve roots run caudally in the subarachnoid space (lumbar cistern) before exiting through their respective intervertebral/sacral foramina. - The filum terminale (pia mater extension) anchors the cord to the coccyx. - Lumbar puncture is performed at L3–L4 or L4–L5 (at the level of the iliac crests, the intercristal line) to sample CSF safely. The needle displaces the mobile roots instead of piercing the cord. - Roots are peripheral nerves (LMN), so injury causes LMN signs. Conus medullaris lesions are different: they are cord lesions and can cause mixed UMN and LMN signs.

**Cauda equina syndrome (CES)** - Causes: central massive lumbar disc herniation (most common, usually L4–L5 or L5–S1), spinal stenosis, tumors (metastasis, ependymoma, schwannoma), epidural abscess or hematoma, vertebral fracture, iatrogenic injury, and ankylosing spondylitis (late). - Clinical features: - Severe low back pain and often bilateral radicular pain (sciatica). - Saddle anesthesia (S2–S4 dermatomes: perineum, buttocks, inner thighs). - Bladder dysfunction: urinary retention with overflow incontinence (early finding is difficulty initiating urination or loss of the urge to void). The detrusor is areflexic. - Bowel dysfunction: fecal incontinence and decreased anal sphincter tone. - Sexual dysfunction (erectile dysfunction). - Asymmetric, flaccid leg weakness, decreased or absent reflexes (knee and ankle jerks), and hypotonia (LMN pattern). - Conus medullaris syndrome vs CES: - Conus: sudden onset, bilateral symmetric, less radicular pain, more perianal sensory loss, early bowel/bladder involvement, mixed UMN/LMN signs (hyperreflexia of the knee, possible Babinski). - CES: gradual onset, asymmetric, severe radicular pain, saddle anesthesia, later bowel/bladder involvement, LMN signs only. - Diagnosis: urgent MRI of the lumbosacral spine (gold standard). Check post-void residual with bladder scan (>200 mL is concerning). - Treatment: emergent surgical decompression (laminectomy/discectomy), ideally within 24–48 hours of symptom onset, to improve bladder, bowel, and motor outcomes. For malignancy: high-dose steroids (dexamethasone), radiation, and/or surgery. For infection: antibiotics and drainage. - Prognosis depends on preoperative bladder status. Retention at the time of surgery carries a worse prognosis.

**Board pearls** - Classic vignette: a patient with back pain after heavy lifting, then urinary retention, saddle anesthesia, and decreased rectal tone, so order an MRI and arrange emergent neurosurgical consult. - Cord compression in cancer (above L1–L2) causes UMN signs, whereas CES causes LMN signs. - A disc herniation usually compresses the root one level below the herniated disc in the lateral (posterolateral) position, but a central herniation can compress multiple lower roots (CES). - Differential: Guillain-Barré syndrome (ascending weakness, areflexia, but usually no sensory level or early sphincter dysfunction), conus lesions, transverse myelitis, and diabetic neuropathy.